Polymers are widely employed to produce functional and decorative articles. These polymeric articles are predominantly produced by a melt forming process, such as injection molding, extrusion molding, extrusion blow molding, calendering, thermoforming, or 3D printing.
Nucleating agents can be used as additives for polymeric melt compositions comprising thermoplastic polymer resins in the manufacture of polymeric articles by the aforementioned melt forming processes. In general, nucleating agents can be used to induce a crystalline structure in a polymer during solidification of a plastic melt. Moreover, nucleating agents can increase the crystallization temperature (Ta) and the rate of crystallization (Rc) of the polymer resin. Such an effect can be commercially useful. As described by Stuart Fairgrieve in “Nucleating Agents”, Rapra Review Reports, Vol. 16, No. 7 (2005), with the use of nucleating agents, it is possible, for example, to increase rates of production of injection molded articles given that, with a higher Tc, less cooling of the mold is required, and, with a higher Rc, an article can be removed sooner from the mold. Fairgrieve (2005) discloses that nucleation of polymer crystallization in commercial processes can be influenced by the addition of “low levels”, which is characterized by Fairgrieve (2005) as generally between about 0.01 and 1 wt %, of certain additives specifically selected for this purpose.
One type of nucleating agent that can increase rates of production for at least some polymeric articles is saturated dicarboxylate salts. This type of compound includes, for example, saturated bicyclic dicarboxylate salts, and, specifically, disodium bicyclo[2.2.1]heptanedicarboxylate, available from Milliken & Company under the HYPERFORM HPN-68 trademark. Compounds of this type are disclosed in, for example, U.S. Pat. Nos. 6,465,551 and 6,534,574 both to Zhao et al. Generally, saturated dicarboxylate salts are designed for use as a nucleating agent for polyolefins such as polypropylene. Additionally, saturated dicarboxylate salts are relatively expensive. As a result, use of saturated dicarboxylate salts may not be technically suitable for polymeric materials other than polyolefins, or cost-effective for lower-cost or commodity-type polymeric materials such as polyvinylchloride.
Other types of additives also can be referred to as “nucleating agents”. However, such additives typically are used for different purposes to produce different types of products than those for which the nucleating agents discussed above typically are used. Namely, such additives that also can be referred to as “nucleating agents” are more precisely used as foaming agents or cell regulators or both in the production of foamed or expanded plastics. In this context, the so-called “nucleating agents” are added to induce and/or regulate the formation of tiny gas bubbles to provide the desired foaming.
For example, U.S. Pat. No. 4,048,208 to Spicuzza, Jr., et al., discloses that expanded polystyrene sheet can be made from a mixture including a nucleating system in an amount from 0.01 to 5%, and preferably, 0.05 to 3%, based on the total weight of polymer. Nucleating systems useful in the invention disclosed by Spicuzza include carbon dioxide, nitrogen, compounds which decompose to produce carbon dioxide or nitrogen, and mixtures which react to produce carbon dioxide or nitrogen, such as mixtures of alkali or alkaline earth carbonates and organic acid (e.g., sodium bicarbonate and citric acid). An exemplified mixture contains 0.2% sodium bicarbonate and 0.2% citric acid together with 5.5% n-pentane, 0.1% hydrophilic silica, 0.1% magnesium oxide, and a balance of extrusion grade expandable polystyrene particles. The mixture is used to produce an extruded expanded sheet product, which is reported as having foam cells with diameters generally in the range of 2 to 3 mils.
As further examples, U.S. Pat. Nos. 4,572,740 and 5,225,107 both to Kretzschmann et al., disclose that it is known to use citric acid together with sodium bicarbonate as a blowing agent or as a so-called “active nucleating agent” that serves as a pore regulator in the production of foamed plastics. U.S. Pat. No. 4,572,740 further discloses the use of 0.4% of a standard commercial blowing agent consisting of sodium bicarbonate and citric acid, and the use of 0.1 to 10% of certain citric acid esters as blowing agents, and the use of 0.05 to 5.0% of certain citric acid esters as nucleating agents, all for the production of foamed plastics. U.S. Pat. No. 5,225,107 further discloses that a citric acid and sodium bicarbonate system is effective as a pore regulator in the production of foamed plastics when added in amounts from 0.4 to 1.0%, and that amounts as low as 0.09% of monosodium citrate and 0.07% of sodium bicarbonate can be used together with 0.25% of talc for nucleating directly gassed thermoplastic foams.
Each of the disclosures of the three aforementioned patent references is limited to the production of foamed plastics and fails to address nucleating agents that are intended for use in the production of unfoamed plastics. That is, these patents do not contemplate the use of nucleating agents for the purpose of inducing a crystalline structure in a polymer, for example, by increasing Tc and Rc such that rates of production can be increased for polymeric articles made from melt forming processes such as injection molding.